i think you get it at mount. pyre
Mt. Pyre in Ruby/Sapphire/EmeraldLost Cave in FireRed/LeafGreenRoute 204 in Diamond/Pearl/Platinum
It depends which kind. You can find them anywhere!
at the top of Mt.pyre (inside)
U look for it
Breed two Marrils, one of each gender, in the day care west of Mauville City. You will also need the item Sea Insence which you can find on Mt. Pyre when dropping through the holes in the floor. I think you just need to make one of the Marrils hold the item. Walk around for a bit and then talk to the man outside of the day care and he will give you an egg that will hatch into an azurill.
That item does not exist in firered.
Diglett's Cave, Kanto Region
how do you think insence is made i dont know
What are the ingredients in cloud nine insence
Step one: Have a male + female Marill/Azumarill. Step Two: Give one of them (doesn't matter which) a sea insence. Step three: Breed.
You can find one on Route 38.
You put rock insence on a sudowoodo and you breed it.